1. Technical Field
The present invention relates generally to process automation for systems which manufacture sheet materials, and more particularly to apparatus and methods for on-line measurement of various quality attributes of such materials, wherein measurement is based on reflectance. Specifically, this invention pertains to apparatus and methods for use in providing measurements of quality attributes wherein the measurements require that two separate determinations of reflectance be made as the sheet material is alternately positioned against dark and light backgrounds.
2. Related Art
It is desirable to know the reflectance of continuously-produced sheet materials for several reasons. In the paper industry for example, the reflectance may be indicative of a variety of quality attributes including gloss, smoothness, opacity, and color to mention a few.
For some quality attributes such as opacity and color, it is desirable to know two reflectance values for the sheet material, one being obtained when the sheet is positioned against a dark background, and the other being obtained when the sheet is positioned against a light background.
The color of the material can be ascertained by illuminating the material with white light and measuring the reflectance therefrom for each of a plurality of wavelength bands in the visible region of the electromagnetic spectrum. Such measurements can be accomplished by using a spectrophotometer such as that disclosed in U.S. Pat. No. 4,076,421 Kishner, for example. However, in the manufacture of paper products, the manufacturer is concerned with the color of the material as it appears to a consumer who observes the material in the form of a wound-up roll or a stack of individual layers as in a book or a napkin. To determine the color (as seen by an observer) of a roll of translucent sheet material such as a thin paper product, it would be necessary to measure the reflectance from a stack of individual plys of the material. This presents a problem in on-line color measurement because typically only a single ply is involved and the observer would perceive a different color if only a single ply of the material were observed. The difference in perception is due to the fact that the opacity of a single ply of translucent material is different from the opacity of multiple plys. Thus, in order to make a color measurement that is meaningful to the manufacturer, it is necessary to account for the opacity of the material. This may be accomplished by providing the forementioned plurality of reflectance measurements twice, once with a dark background positioned against the material to obtain the "dark reflectance", and once with a lighter background positioned against the material to obtain the "bright reflectance", and using known formulae to calculate the opacity-compensated color. Normally, a spectrophotometer or other instrument remains in one location on one side of the moving sheet material while a mechanical device shuttles the dark and light backgrounds successively into the radiation path and against the other side of the sheet so that sequential measurements are made. This arrangement is undesirable for a number of reasons including differential dust buildup on the dark and light backgrounds, and the need to manufacture and maintain cumbersome mechanical devices.
A recent attempt to circumvent this arrangement uses the idea that since multiple plys of the material best represent the desired background, that is precisely what should be provided (See U.S. Pat. No. 4,715,715.). However, while this idea is admirable in its simplicity, it appears not to account for the fact that color changes occur within a single run of material. Moreover, it appears to require a change in background between successive runs, thus adding to the papermaker's duties.
Accordingly, there is a need for a device for measuring dark and bright reflectances while providing alternating backgrounds in a fixed position with respect to the source of illumination and, more specifically, for such a device that obviates mechanical parts while providing uniform standards for the alternating backgrounds.